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Multiple pregnancy

Last updated: July 6, 2021

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Multiple pregnancy refers to pregnancy with two or more fetuses. Twin pregnancies can be differentiated into monozygotic and dizygotic pregnancies. Monozygotic twin pregnancies result from the division of the fertilized oocyte into two embryonic layers, whereas dizygotic twin pregnancies arise from the fertilization of two oocytes with two spermatozoa. Multiple pregnancies are classified based on how the amniotic sac and placenta are divided among the fetuses, which is determined via ultrasound. In monozygotic twin pregnancies with a shared placenta (monochorionic), twin-to-twin transfusion syndrome is a possible complication. In these cases, blood vessels are shared between the twins, with one twin transferring blood (donor) to the other twin (recipient). Twin-to-twin transfusion syndrome is associated with various risks, including anemia and growth retardation in the donor twin, and polycythemia in the recipient twin. Pregnancies with more than one fetus are generally considered high-risk pregnancies, increasing the likelihood of almost all potential complications of pregnancy, including preterm labor, pregnancy-induced hypertension, and preeclampsia.

  • The frequency of multiple births is calculated in accordance with Hellin's law. [1]
    • Twins: ∼ 1:89
    • Triplets: ∼ 1:892 (1:7,921)
    • Quadruplets: ∼ 1:893 (1:704,969)
  • The incidence of multiple pregnancies (particularly dizygotic) has increased since the 1980s as assisted reproductive technology has become readily available

Epidemiological data refers to the US, unless otherwise specified.

Predisposing factors [1]

Monozygotic vs. dizygotic twins

Comparison of monozygotic vs. dizygotic twins
Identical twins (monozygotic twins) Fraternal twins (dizygotic twins)
  • Division of the fertilized oocyte into two embryonic layers
Genetics of the individual
  • Genetically identical
  • Genetically different
Chorionic cavity and amniotic sack
  • Varies (see below)

Special features in the development of monozygotic twins

Overview of different types of monozygotic twin pregnancy
Description Time of division of the zygote Frequency in monozygotic twins
  • ∼ 20–30%
  • The twins share a single chorionic sac (the twins share a placenta) but have a separate, individual amniotic sac.
  • ∼ 70%
  • ∼ 1–5%

Monochorionic-monoamniotic (conjoined twins)

  • < 0.1%

A four-wheeler has SPACe for twins.” 1st four days (0–3): Separate placenta and amniotic sac; 2nd four days (4–7): shared Placenta; 3rd four days (8–11): shared Amniotic sac; day 12 and beyond: Conjoined twins.

Physical examination [1]

Ultrasound [1]

Almost all complications associated with normal pregnancies are more likely in multifetal pregnancies.

Maternal illnesses [1]

Fetal illnesses

We list the most important complications. The selection is not exhaustive.

Prenatal care [1]


  1. Multiple Pregnancy . Updated: July 1, 2015. Accessed: April 27, 2017.
  2. Bamberg C, Hecher K. Update on twin-to-twin transfusion syndrome. Best Practice & Research Clinical Obstetrics & Gynaecology. 2019; 58 : p.55-65. doi: 10.1016/j.bpobgyn.2018.12.011 . | Open in Read by QxMD
  3. Multifetal Pregnancy Reduction.